1. Field of Invention
The present invention relates to a tensioner for a chain or belt and more particularly, but not exclusively, to a tensioner for imparting tension to a chain or belt used in a timing drive of an internal combustion engine of a vehicle.
2. Description of Related Art
Internal combustion engines of motor vehicles often include a timing belt or chain drive that passes over sprockets on the crankshaft and camshaft and is used to ensure that the camshaft is driven synchronously with the crankshaft. The tension in such a chain or belt varies considerably as a result of the expansion and contraction of engine components with temperature, torsional vibrations imparted from the crankshaft and camshaft, the engine speed and chain elongation as a result of chain wear or temperature variations in chain components. It is important to impart to and maintain tension in the chain or belt so as to reduce noise and the likelihood of the chain or belt jumping from the teeth of the sprockets.
Tensioners for chains or belts generally comprise a housing that defines an open-ended cylinder in which a plunger is slidably movable in a longitudinal direction and is biased outwardly of the cylinder by a coil spring so as to impart tension to the chain or belt. A variable volume fluid pressure chamber is defined between a hollow in the plunger and the cylinder walls. A check valve permits hydraulic fluid to pass from a source such as an oil pump into the pressure chamber but prevents passage of fluid in the reverse direction. The fluid in the pressurised chamber also serves to bias the plunger out of the cylinder towards the chain or belt.
The combined effect of the hydraulic fluid pressure and the coil spring biasing forces moves the plunger out of the housing and into contact with the chain or belt so as to take up the slack. An opposite resisting force is imparted from the chain to the plunger as a result of the tension induced in the chain by the drive. If the chain or belt is subjected to an increase in tension the resulting force applied to the plunger attempts to move it into the cylinder. However, such movement is prevented by the check valve which prevents the escape of hydraulic fluid out of the chamber. The incompressible nature of the hydraulic fluid prevents instant return movement of the plunger although a small annular clearance between the plunger and the cylinder wall may permit some fluid escape and limited slow retraction of the plunger.
When the internal combustion engine is idling or at rest there is little or no hydraulic fluid pressure and with insufficient fluid pressure in the chamber it is easier for the plunger to retract into the cylinder and for vibrations and noises to be transmitted from the chain. In order to address this problem it is common to provide a locking ratchet mechanism to prevent excessive retraction of the plunger even when the fluid pressure is low.
Known ratchet mechanisms for chain or belt tensioners comprise a rack defined on the plunger that is engageable by a pawl on the cylinder as described in U.S. Pat. Nos. 4,822,320 and 5,073,150. In an alternative embodiment a cylinder is provided with a helical toothed channel that is engageable by a pin on the plunger. The disadvantage of such ratchet mechanisms is that they are susceptible to poor performance and even jamming as a result of slight axial misalignment of the plunger in the chamber caused by the locking force is being applied on side only. Moreover, the components require careful machining that is relatively expensive to perform.
The tensioner described in U.S. Pat. No. 4,792,322 has a ratchet locking mechanism in which the locking force is applied uniformly around the plunger. It relies on the use of a radially resilient ring that moves between annular notches defined at axially spaced intervals in the interior wall of the cylinder. The ring serves to prevent excessive movement of the plunger into the cylinder by becoming trapped between an edge of the notch on the cylinder wall and a shoulder defined on the plunger. As the plunger extends out of the cylinder the ring is forced to move with it by contact with a stop face on the plunger whereupon it is moved into the next notch in the cylinder. The arrangement prevents retraction of the plunger but only to a predetermined number of discrete positions governed by the number of notches and their spacings. Like the other aforementioned ratchet designs, the locking of the plunger against retraction at discrete positions does not always ensure the appropriate tension is imparted to the chain.
U.S. Pat. No. 5,704,860 describes a chain tensioner having a radially resilient stop ring that co-operates with grooves in the wall of the plunger and housing. The ring is clamped between two contact surfaces on the housing and piston to lock the piston in the housing bore despite the urging force of a compression spring. If the piston is pushed into the housing the stop ring snaps into a groove so as to allow limited movement of the plunger in the housing. However, it does not move entirely clear of the plunger and further extension of the plunger encourages the stop ring to move along the housing.
EP 0260565 describes a chain tensioner in which a locking ring is radially resilient so that it is biased on to the plunger. It is moveable into a predetermined position where it is squashed between contact surfaces on the piston and housing and prevents extension of the plunger. However, it does not move clear of the plunger.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages.